Discriminator circuit



Jan. 8, 1952 c; J. NORTON 2,581,968

' DISCRIMINATOR CIRCUIT Filed Aug. 8, 1947 Fig. 2

m.- MIXER -'I.F. 2ND AUDI-O LOUD AMPLIFIER IST. DETECTOR AMPLIFIERDETECTOR AMPLIFIER SPEAKER I LOCAL 0:50am OSCILLATOR INATOR CONTROL Figrjf- F F w- 5 INVENTOR- Clyde Norton BY M ATTORNEY Patented Jan. 8,1952- DISCRIMINATOR CIRCUIT Clyde J. Norton, Braintree, Mass, assignorto -Sylvania Electric Products Inc., Salem, Mass.,

a corporation of Massachusetts Application August 8, 1947, Serial No.767,588

2 Claims.

j"'1 his"invention' relates to electric control circuits of thediscriminator type for developing acontrol potential-proportional to thedeviation of'an input signalffrom a desired frequency.

The circuit of the invention is arranged to receive "'a' radio frequencysignal which is fed to a network of impedance and'capacitance unitshaving predetermined characteristics. The network 'includes two parallelcircuits through which the input signal passes. A connection is madefrom between the elements of each parallel ciredit to a pair of 'diodetubes connected for rectification 'at opposite polarities. 'The diodeoutputs are 'fedtofa mixer circuit from where the developed controlsignal is fed to a utilization circuit.

In one of the parallel circuits, signals above the desired frequencydevelop a certain poten-' tial at the connection between its elementswhen the frequency is higher than desired while at the connectionbetween elements of the other parallel circuit a lower potential'isdeveloped. These two radio frequency potentials when rectified by thediode tubes and passed through a mixer means appear as a controllingvoltage of polarity and magnitude determined by the deviation of thefrequency ofgthe input signal from the desired frequency. This controlvoltage is used through a frequency changing device to correct anydeviation from the desired frequency which appears at the input. a

It is an object'of the invention to provide a discriminator circuitwhich is highly efficient and accurate in operation.

A further object of the invention is to provide a discriminator circuitadapted to accept and correct a signal which varies a substantial amountfrom the desired output frequency of a utilization circuit.

A further object of the invention is to provide a discriminator circuitcomposed of a minimum of elements which are extremely stable inoperation.

A further object of the invention is to provide a discriminator circuitwhich requires a minimum of power input energy for its operation.

To provide a better understanding of the invention a particularembodiment thereof will be described and illustrated in the accompanyingdrawings in which i Figure 1 is a diagrammatic view of a circuit vembodying the invention.

Figure 2 is a block diagram illustrating one application of theinvention.

[Figures 3 and 4 illustrate graphically the improved operation of thecircuit of the invention. The invention may be utilized in manydifierent ways. For example, it may be applied to a superheterodynereceiver as shown in block diagram in Figure 2. The invention relates tothe discriminator of the radio receiver which takes a portion of theinput signal from the intermediate frequency amplifier, performs itscorrective functionand passes a correcting signal to a control-' lingelement which in turn conducts a signal 1 to a local oscillator fromwhere .a corrected frequency component goes to the first detector mixer.From here a corrected signal is transmitted to the intermediateamplifier, the second detector and thence to the audio system of thereceiver.

As indicated the invention resides in the discriminator circuit which isshown diagrammatically in Figure 2. A radio frequency signal is fed tothe input circuit I from where the signal I passes through a pair ofparallel circuits 2 and ductance 9. It should be noted that the inductance and capacitances in the circuits 2 and are connectedrespectively'in reverse order with respect to conductor I, which isatground potential.

A connection [0 is made between the elements of circuit 2 and connectedto the cathode ii of a diode tube l2 through a conductor l3. Anotherconnection I4 is made from between the elements 8 and 9 to the plate [5of another diode tube I6 through a conductor H.

The cathode I8of the diode l6 and the plate IQ of the diode l2 areconnected to a mixer'network comprising aresistor 20 to the ends ofwhich the cathode l8 and the plate [9 are connected. A center tap 2| onthe resistor '20 is connected to ground through a suitable resistor 22.The cathode l8 and the plate [9 have connected thereto respectivelycondensers 23 and 24 which are in turn connected to ground. Thus I anyalternating current appearing at the diode turn'path to the groundedinput terminal to which the input signal is applied. Cathode l5, it willbe understood, has a direct current return circuit to this groundedinput terminal through inductive element 9.

In operation a radio frequency signal is impressed upon the inputconnections i which are 'quency equal potentials appear at points it andi l and therefore equal potentials appear at the p te iian heflca elsifihe die s it and I2. Since the outputs of the diodes are opposite inpolarity and equal in potential the resents the desired (center)frequency while the resulting potential at the center tap of the mixerunit is zero. No potential therefore passes along through the outputlead and no correctiod takes place.

than the desired frequency the impedance of the inductance d in circuit2 becomes higher thus reducing the potential at the connection wandtherefore at the cathode i I. At this same relatively higherfrequencyf'jthe signal in passing through the circuit 3 meetsarelatively lower impedance in the condenser 8 thus'a'potential nowcreates an unbalanced condition at the cenj ter tap of the mixerresistor 28. Thus a plus or minus voltage appears in the output'lead2:3; This voltage'is'ofa polarity and'valne in proportion to thefrequency correction required.

This correction potential then 'passesto "the control device and'thenceto the local oscillator which'corrects the, input'signal at thefirst detector and mixer unit.

The above result takes place when a signal is received having; afrequency higher than desired. If a signal of lower frequency isreceived,

the impedance of the inductance 4 thereto will i be relatively lowerthan to a signalojf the desired loop, said elements. be ing'proportionedto yield frequency. Therefore the potential of the point it! will berelatively higher. On the other hand the impedance of the condenser 8"now'becomes greater witha consequent drop in potential at the point It.It follows therefore that av correction potential now appears in theconductor '23 having a polarity opposite to that which appeared to thedesired frequency. By making the capacitors 5' and 8 variable the twocircuits 2 and 3 are. 'eadily tuned to the desired frequency. Moretiverif it is desired tov change the desiredcorrec'ted frequency 'thism'aybejdone by adjusting the ondensersjto resonancefatthe new frequency.

An important feature of the present, invention lies in its ability tocorrect an input signal which deviates a relatively great amount fromthe desired frequency. discriminators in use before the presentinvention.

Figure 3 is a curve representing the scope of acceptance ofoff-frequencysignals in a conventional discriminator] circuit. Vertical'coordinates represent output voltage (that can be used for tuningcorrection in proper organi'za while horizontal coordinates represent;frequency The place where the curve crosses tions linet'! represents thedesired or center frequency.

The curve shows that very little frequency cor This has not been true ofFL 11, however, the input frequency is higher rection voltage can beexpected outside the immediate center of the discriminator characterisfiOL I lligliie te ,ss wst academy characteristic of the discriminatorcircuit embodying the invention. The place where the curve crosses line28 rephorizontal-portions 29 of the curve illustrate the relativelylarge voltage available for frequencycorlfe n. qutside; the immediatecenter range where.theoutputvoltage is a linear function of frequencydeviation. What f'claimisxdiscriminator circuit comprising two inductiveelements and {two capacitive elements connected, in alternation in aloop. an opposite pair of connections in sa' constituting input.

terminals and the re" ammg'pair of connecdons. constituting, .utputermmns of said loop. said elements bei Drop r'tioned to yield equalvoltage between e1 her a: ut terminal .andone of "the input terminals ata'desired frequency. a diode having its anode connected to one of saidoutput terminals. a. second diode having. its cathode connectedj'to t1)other output, terminal, a pair of condensers 'ected'in series to theremaining electrodes of jsaid'diodes, the junction ofsaid"condensers:lieimg connected to said one input terminal; a, pairofequai. resistors also connected between said remaining, electrodes ofsaid diodes, and a load resistor between the'junction of said equalresistors and said condensers, a discriminator output'bein'g availableacross said load resistor.

2. A discriminator circuit comprising. two inductive elementsandftwo'capacitive elements. connected inalternation. a. loop, an.opposite pair of connectionsin said'loop' constituting input terminalsand' the remaining'pair of connections constituting output 'terminals ofsaid having its cathode connected to. the other output terminal, a pairOi,- condensers connected in series to the remaining electrodes, of saiddiodes. the junction of said condensers being, connected toone of saidinput terminalsa pair, of. equal resistors also connected between saidremainin electrodes of said dio des, afload resistor between thejunction of said equal resistors, and said condensers. a discriminatoroutput being available. across said load resiston, one of saiddiodeshaving a direct curreiniv path through one of said inductiveelements,insaid1oop, .'to said one input. terminal, and a. resistorbetween said one. input terminaland the connection. of the other diodeto the loop for providinga direct current path.

REFERENCES; crrisn The following references are of-rrecord-in the fileof thispatent;

UNITED STATES 1 BAIENI'S.

